SYDNEY (Reuters) - The United States and Australia plan to push the United Nations to adopt international standards to deal with growing numbers of foreign fighters in Middle East conflicts and the threat they could pose when they return to their home countries.

U.S. Secretary of State John Kerry said on Tuesday he would take the issue to the United Nations later this year, calling for the adoption of a “best practice” that all countries can follow to reduce the threat.

“We have a responsibility to take this to the United Nations and to the world, so that all countries involved take measures ahead of time to prevent the return of these fighters and the chaos and havoc that come with that,” Kerry told reporters in Sydney.

Security analysts have put the number of foreign fighters in Iraq and Syria, traveling from scores of countries around the world, in the thousands.

Australia, which believes at least 150 of its citizens are involved in fighting or actively supporting the Islamic State (ISIL) in Syria and Iraq, has been spearheading a push to deal with the issue.

Australian Foreign Minister Julie Bishop said it “truly poses one of the most significant threats we’ve seen in a long time”.

“Our fear is that they will return home to Australia as hardened homegrown terrorists and continue their work here in Australia,” she told reporters in a joint news conference with Kerry following annual Australia-United States Ministerial Consultations in Sydney.

The issue gained even greater prominence at the talks following the publication on Monday in Australian media of a photograph of a young boy holding the decapitated head of a Syrian soldier.

‘GROTESQUE’

The image was posted on Twitter and showed the boy, believed to be the son of Australian jihadist Khaled Sharrouf, The Australian newspaper said, adding that the boy was aged 7.

The image was taken in the northern Syrian city of Raqqa and was posted last week on the Twitter account of Sharrouf, Australia’s most wanted militant who fled to Syria last year and is now an Islamic State fighter.

“This image, perhaps even an iconic photograph ... is really one of the most disturbing, stomach-turning, grotesque photographs ever displayed,” Kerry said. “This is utterly disgraceful and it underscores the degree to which ISIL is so far beyond the pale.”

Bishop said a number of countries at a meeting of foreign ministers of the 10-member Association of Southeast Asian Nations in Myanmar at the weekend had raised the issue of foreign fighters in the Middle East and the prospect of them returning home with militant skills and experience.

Australia is considering a series of legislative reforms, including forcing people to face the legal burden of proof to explain what they are doing in areas proscribed by the Australian government and the possibility of suspending passports to investigate activities of people in Australia.

“This threat is so real,” said Kerry, recounting how the president of an unidentified north African country recently told him that it had 1,800 identified citizens who had gone to fight.

About 1,100 of those had been killed, he said, leaving some “700/800 who they fear are going to return to that country knowing how to fix an IED, knowing how to arm weapons, knowing how to explode a bomb, knowing how to build a suicide vest.”